1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for controlling pressure and measuring flow. More specifically, embodiments of the invention generally relate to a method and apparatus for controlling gas provided between a substrate and a substrate support in a semiconductor processing chamber or to a semiconductor processing chamber.
2. Description of the Related Art
Substrate temperature is an important process control attribute critical to many microelectronic device fabrication processes. Providing gas between the substrate and a substrate support in a semiconductor processing chamber is a well-known method for improving heat transfer between the substrate and the substrate support, thereby enhancing the precision and uniformity of substrate temperatures.
FIG. 1 depicts a simplified schematic of a conventional semiconductor processing chamber 150 having a gas delivery system 100 shown providing backside gas between a substrate 154 and a substrate support 152 disposed in the processing chamber 150. The processing chamber 150 may be configured to perform chemical vapor deposition (CVD), physical vapor deposition (PVD), etch chamber or other vacuum processing technique. Process gas delivery systems, pumping systems and the like for controlling processes performed within the processing chamber are well-known and have been omitted for the sake of brevity.
The substrate support 152 generally includes a passage 156 formed therethrough for delivering a heat transfer gas (hereinafter referred to as backside gas) to an area 158 defined between the substrate 154 and substrate support 152. The size of the area 158 has been exaggerated for clarity. The backside gas, such as helium or another gas is generally provided by the gas delivery system 100.
The gas delivery system 100 located outside the processing chamber 150 and includes a gas supply 104 and control circuit 102. The delivery of backside gas from the supply 104 to the area 158 is regulated by a control circuit 102. A shut-off valve 106 is generally provided between the supply 104 and control circuit 102.
The control circuit 102 generally includes a thermal flow sensor 110, control valve 112, a pressure sensor 114 and a restrictor 118. An inlet line 120 is coupled to an inlet of the flow sensor 110 and facilitates coupling the control circuit to the shut-off valve 106. A first intermediate line 122 couples an outlet of the flow sensor 110 to the control valve 112. A second intermediate line 124 couples an outlet of the control valve 112 to an outlet line 126. The outlet line 126 facilitates coupling the control circuit 102 to the passage 156 to that gas provided by the supply 104 may be delivered in a regulated manner to the area 158 between substrate 154 and substrate support 152. A pressure sensor 114 is coupled to the second intermediate line 124 and is adapted to provide a metric of pressure of the gas within the second intermediate line 124.
A bypass line 128 is teed into the outlet line 126 and is coupled to a vacuum source 116. A restrictor 118, such as a needle valve, is provided in series with the bypass line 128 to regulate the flow therethrough.
In operation, the control circuit 102 is set to a predefined pressure measured by the pressure sensor 114. The flow sensor 110 measures the flow of gas to the control valve 112. The control valve 112 is modulated in response to pressure variations as detected by the pressure sensor 114, such that the pressure of gas delivered to the area 158 between the substrate 154 and the substrate support 152 is provided at a predefined pressure.
Although this design has proven to control pressure in this application, field experience with the existing technology has increased the demand for more accurate measurement of flow. In addition accelerated response to change in pressure set points is needed to reduce process cycle times. For example, gas temperature and/or pressure fluctuations upstream of the gas delivery system may make the flow through the flow sensor unstable, thereby reducing the accuracy of the correlation between the flow indicated and the actual flow to both the area between the substrate and substrate support and the restrictor. Additionally, variation in the vacuum provided by the vacuum source may impact the flow through the restrictor, which may falsely indicate or contribute to erroneous interpretation of the amount of gas disposed between substrate and substrate support. In critical applications, the gas available as a heat transfer medium between the substrate and substrate support may vary, leading to deviation in substrate to substrate process performance.
In addition, the system as described in FIG. 1 is unable to determine the rate of gas flowing into the area between the substrate support and substrate or to determine small variations in the rate of gas leakage between the substrate support and substrate that may cause the heat transfer characteristics and uniformity to vary, thereby resulting in unwanted variation in processing performance. Thus, it would be desirable to know in addition to pressure the rate of gas flow to the substrate support.
Therefore, there is a need for an improved method and apparatus for controlling the delivery of backside gas in a semiconductor processing system.
Chamber pressure control is an equally important process control attribute. Throttle valves are typically placed between the chamber and a vacuum pump-to control chamber pressure. In these applications a chamber pressure gage provides feedback to the throttle valve controller. However in an application where the conductance between the throttle valve and the chamber is much smaller then the controllable conductance of the throttle valve, it is not possible to control chamber pressure with a throttle valve between the chamber and a vacuum pump. Therefore, there is a need for a method and apparatus for controlling the delivery of gas into a chamber such that the delivery rate results in the desired chamber pressure.